This invention relates generally to electronic circuits incorporating fuses coupled in series, and the preferred embodiment relates more specifically to protection circuits in battery charging circuits incorporating series fuses for circuit protection.
Electronic circuits often employ fuse elements for protection. When too much current runs through a fuse, i.e. a current in excess of the rating, the soft fuse material vaporizes, thereby opening the circuit. For example, a hair dryer may include a fuse that protects the hair dryer from over-current damage. If one accidentally drops a hair dryer in a sink, the water may create a path for excessive current to flow. This could cause the dryer to stop working. To avoid damage to the dryer, a fuse in series with the power cord will clear, thereby disconnecting the hair dryer from the wall outlet.
Rechargeable batteries often use fuses to protect the cell. When rechargeable batteries, like those made with lithium, if they are overcharged they can release gasses at high temperatures. When such a situation occurs, reliability of the battery may be compromised.
Consequently, rechargeable battery pack manufacturers often place fuses (in addition to battery protection circuits) in the battery packs. If a temporary problem occurs, the battery protection circuit, which includes transistor switches, will open the circuit to prevent an over-current situation. However, in the event that a catastrophic condition of battery protection circuit failure occurs, the fuse will clear thereby providing another layer of protection.
The problem with such fuses is that they are often non-replaceable. In other words, manufacturers often solder them to a circuit board that is inaccessible to the end user. Thus, when the fuse clears, the battery is protected, but it is also rendered useless as the battery cell is now disconnected from the terminals of the battery pack. Occasionally, a short current spike may cause the fuse to inadvertently blow. This is known as xe2x80x9cnuisance trippingxe2x80x9d. If a nuisance trip occurs, the user must throw the battery away and buy another one, even though the cell and circuitry may be imperfect condition. With the cost of some batteries for cellular phones exceeding one-hundred dollars, there is thus a need for an improved protection circuit to prevent nuisance tripping in the series fuse.